X-ray imaging devices are widely used in the fields of diagnostic imaging and industrial inspection alike. For measurement of an X-ray signal that exponentially decays in a subject, a measurement system that offers a wide dynamic range is needed. In general, the maximum value of the dynamic range is determined with a maximum output capacity given by an X-ray source and a maximum permissible dose of a subject. Based on a maximum incident dose attained under these conditions, the maximum detectable capacity of an X-ray detector is designed. On the other hand, the minimum value of the dynamic range is determined with noise caused by an X-ray detector. For expansion of the dynamic range, minimization of noise is essential.
The noise includes circuit noise (internal noise) occurring in the detector and noise derived from mixing of electromagnetic waves coming outside the detector (external noise). In particular, since an X-ray imaging device includes numerous sources of intensive electromagnetic waves such as an X-ray generator, a power source for the X-ray generator, a mechanical system, and others, the latter external noise often becomes a major factor of a decrease in the dynamic range. Therefore, an electromagnetic shield has been used to shield the X-ray detector or any other measures have been taken (refer to, for example, patent documents 1 and 2).
Patent document 1: Japanese Patent Application Laid-Open Publication No. 2002-595
Patent document 2: Japanese Patent Application Laid-Open Publication No. 2004-325126